For example, in the case of fill level measurement apparatuses, such devices—also designated as vibronic sensors—have e.g. an oscillation fork, a single rod or a membrane as a vibration-capable unit. In operation, this vibration-capable unit is excited to mechanical vibrations by means of an electromechanical transducer unit, which may in turn by a piezoelectric actuator or an electromagnetic actuator, for example. In the case of flow rate measurement apparatuses, however, the vibration-capable unit may also be designed as a vibration-capable tube through which the respective medium flows, for example in a measurement apparatus operating according to the Coriolis principle. Naturally, it is understood that additional possibilities which likewise fall under the present invention are also present in addition to the cited examples of a vibration-capable unit and an electromechanical transducer unit.
Corresponding field apparatuses are manufactured in a wide variety by the applicant and—for example in the case of fill level measurement apparatuses—are distributed under the designation LIQUIFANT and/or SOLIPHANT. The underlying measurement principles are known in principle from a plurality of publications. The excitation of the vibration-capable unit to mechanical vibrations by means of the electromechanical transducer unit for the most part takes place via an analog electrical oscillating circuit. The electromechanical transducer unit excites the vibration-capable unit to mechanical vibrations by means of an electrical excitation signal. Conversely, the electromechanical transducer unit may receive the mechanical vibrations of the vibration-capable unit and transduce them into a received electrical signal. The electromechanical transducer unit accordingly comprises either a separate actuator unit and a separate receiver unit, or a combined actuator/receiver unit.
The actuator/receiver unit is thereby part of a control circuit integrated into an electronic unit, which control circuit adjusts the excitation signal such that a predeterminable phase shift is present between the excitation signal and received signal. For example, the oscillating circuit condition, according to which all phases occurring in the oscillating circuit result in a multiple of 360°, must be satisfied for a resonant vibration.
Both the excitation signal and the received signal are characterized by their frequency, amplitude and/or phase. Changes in these variables are accordingly typically used to determine the respective process variable, for example a predetermined fill level of a medium in a container, or also the density and/or viscosity of a medium, or the flow rate of a medium through a tube. In the case of a vibronic point level switch for fluids, for example, a differentiation is made as to whether the vibration-capable unit is covered by the fluid or vibrates freely. These two states—the free state and the covered state—are thereby differentiated using different resonance frequencies, i.e., a frequency shift, for example. The density and/or viscosity in turn can be determined with such a measurement apparatus only given an at least partial coverage with the medium.
In the prior art, both analog and digital methods for the excitation of the vibration-capable unit have been known, wherein the digital excitation is characterized by its more universal possibilities for use. However, this in turn often disadvantageously involves a markedly higher power consumption for the respective measurement apparatus. Therefore, a digital excitation method with a low power consumption would be desirable.
For example, in German Patent, DE102009026685A1 a method has become known for digitally controlled excitation of a vibronic sensor which is based on a forced excitation with a defined frequency. In order to find the excitation frequency for the excitation signal at which the predeterminable phase shift is present, a frequency sweep is implemented, and the frequency corresponding to the predeterminable phase shift is determined. An advantageous development of this method is the subject matter of German Patent, DE102009028022A1, in which the evaluation of the received signal is simplified in that the received signal is sampled and evaluated phase-selectively only at specific points in time. An additional further development is described in German Patent, DE1020110075113A1 and lies in conducting two frequency sweeps in different travel directions with subsequent mean value calculation, in order to increase the measurement precision. However, measurement apparatuses which are designed to execute the cited methods are not, without additional steps, suitable for operation of the measurement apparatus via a 4-20 mA interface or a NAMUR interface.
An additional digital possibility for a vibronic sensor to regulate the phase shift between the excitation signal and received signal at a predeterminable value is disclosed in German Patent, DE00102010030982A1. The method described there is based on the functional principle of a phase control loop (phase locked loop, PLL), and is already optimized for a reduction of the power consumption. For such an arrangement, at least one phase detector is required which has a decisive influence on the robustness as well as on the precision of the control loop. So that the evaluation may take place stably, it must additionally be ensured that the amplitude of the excitation signal is kept to a constant value. However, this is comparably elaborate in practice.